Many gases of all kinds are employed presently in semiconductor manufacturing units, etc. .
For example, general gases such as Ar, He, O.sub.2, H.sub.2, N.sub.2 and the like, are employed including various reactive gases such as Cl.sub.2, CCl.sub.4, SiH.sub.4, SiCl.sub.2, H.sub.2, SiCl.sub.4, BF.sub.3, PH.sub.3, AsH.sub.3, CF.sub.4, BCl.sub.3 and CH.sub.2 F.sub.2 or the like. These gases are increasingly required to be highly pure with the progress of high integration of LSIs, and ultrahigh purification of raw material gases and of gas supply systems has been brought into realization with recent development of diverse techniques associated therewith. However, in order to prevent any particle produced from raw material gases and a gas supply system from invading into a reaction space of each apparatus, it is general to insert filters into the gas supply system everywhere.
While, these filters, although having to some degree performance to remove fine particles involved in those gases, are likely to gradually release gases absorbed therein in processes of manufacture, cleaning, and assembly, etc., of semiconductor devices. These gases act as sources of contamination. FIG. 6 shows experimental data concerning this problem. FIG. 6(a) is a schematic of an experimental device for measuring degassing of a filter, in which the filter to be examined is mounted on an ultra-high vacuum chamber through a valve and one side of the filter is sealed. The same figures (b), (c), and (d) respectively illustrate results of examination by this device of the rate of degassing of a membrane filter made of Teflon prevailing as a gas filter. Hereupon, the same figure (b) illustrates a change in the degree of vacuum, and the same figures (c) and (d) respectively show measured results by a quadripolar mass filter of changes in partial pressures of gas components having large mass numbers comprising the atmosphere and hydrocarbon in the main. The measurement was effected for 32 hours at room temperature (20.degree. C.) and for 30 hours after heating the filter to 110.degree. C., and thereafter for 12 hours after returning it to room temperature. As evidenced from the same figure (c), the Teflon-made membrane filter releases gases of the components of the atmosphere to a high degree even at room temperature before baking, and further releases the atmospheric components in large quantities also after baking thereof for 30 hours at 110.degree. C. for degassing thereof. The same figure (d) is more important. This shows a result of measurements on the characteristics of degassing of the filter for large mass number hydrocarbons when the filter is baked for 30 hours at 110.degree. C. and thereafter returned to room temperature. As clearly evidenced from the characteristics, some degassing is, as a matter of course, found upon the baking, but even after the degassing there are existent released gases of about from 10.sup.-9 to 10.sup.-10 Torr.
These gases of the hydrocarbons are considered to originate from adsorption of some organic solvents etc. employed in a process of cleaning of the filter.
The reason of such release of abundant gases is that the concerning filter itself is arranged to have a very large effective surface area and the Teflon surface thereof is structured to have innumberable fine holes, whereby those gases are adsorbed and stored on those surfaces, and freed therefrom. With such a filter mounted on a piping system, even if the piping system is constructed with a high purity material, released gases from that filter contaminate high purity gases and as a result those gases so contaminated are fed to a semiconductor manufacturing unit.
For example, when gases such as H.sub.2 O, etc., mix into Ar gas to be fed to a RF sputtering unit, which serves to make a film from metal such as aluminium, it causes the surface of an Al target being sputtered to be oxidized with ease by H.sub.2 O involved in the atmosphere to form Al.sub.2 O.sub.3 (alumina) on the target surface because it is very active. Since the rate of sputtering for Al.sub.2 O.sub.3 is reduced compared with Al, the rate of sputtering for the target reduces and thereby the rate of film making decreases sharply. In addition, such H.sub.2 O mixed into the Ar gas is also incorporated into the resulting Al film, whereby resistance of wiring of Al is increased to reduce reliabitity to electromigration. Moreover, when such H.sub.2 O is introduced into the RIE unit, active species O and OH are produced in the plasma atmosphere, and when polysilicon is rendered to etching for example, SiO.sub.2 is formed on that surface to result in uneven etching with a reduced ratio of selection between the polysilicon so rendered to etching and a underlying SiO.sub.2 film.
In view of the drawbacks of the prior techniques, it is an object of the present invention to provide an apparatus capable of supplying ultrahigh purity gas by using a filter in a gas piping system.